Molecular dynamics simulation of chains mobility in polyethylene crystal

Abstract

The mobility of polymer chains in perfect polyethylene (PE) crystal was calculated as a function of temperature and chain length through Molecular dynamics (MD) in united atom approximation. The results demonstrate that the chain mobility drastically increases in the vicinity of the phase transition from the orthorhombic to quasi-hexagonal phase. In the quasi-hexagonal phase, the chain mobility is almost independent on temperature and inversely proportional to the chain length.Mechanical properties of semi-crystalline polymers below the melting temperature are strongly governed by morphology of crystallites that depends on how the melt was prepared and treated. Polymer crystallites can undergo a variety of structural phase transitions that is a subject of extensive studies. PE is widely used as a 'model system' for high-crystallinity polymer [1]. At normal conditions (room temperature and atmospheric pressure), the PE crystal is in orthorhombic [2] or monoclinic phase [3]. The PE crystals undergo phase transition to quasi-hexagonal phase at elevated pressure (more than 400 MPa) and temperature (> 520K) [4][5][6].Quasi-hexagonal phase has been found in numerous diverse polymeric systems [7]. One common property of the quasi-hexagonal polymeric phase is some degree of conformational disorder, either in the main chain or in the side groups or in both. Compared with the ordered crystalline state, there is a high degree of molecular mobility, with the chain performing both rotational and translational motion. Translational chain mobility enables easy formation of extended chain crystals in polymers that exhibit the quasi-hexagonal phase; isothermal extension of the initially folded chains has been shown to occur in the quasi-hexagonal phase [7]. The PE chains in both monoclinic and quasihexagonal crystals are parallel to each other. However, the monomers of the chain in monoclinic phase predominantly belong to the same plane, while the monomers of the chain in quasi-hexagonal phase are randomly oriented. Therefore, the chains in quasi-hexagonal phase form close packing of rods [7].Diffusion rate of PE chains in quasi-hexagonal phase was measured experimentally in Ref.[6] using proton spin-lattice relaxation experiments and significant increase in the rate of chain diffusion from about 10 −12 cm 2 /s (orthorhombic phase) to 10 −9 cm 2 /s (quasi-hexagonal phase) was observed. The molecular dynamics modeling and understanding of the atomistic mechanisms of high chains mobility in the quasi-hexagonal phase is the purpose of the current work.The LAMMPS software package [8] was utilized for molecular dynamics simulations. Polyethylene chains were modeled in united atom version of Dreiding forcefield [9]. The Nose-Hoover style thermostat and barostat were used in these calculations [10]. The modeled samples were comprised of 64 polyethylene chains with 36, 100 and 200 carbon atoms in each chain. The simulations were performed in periodic boundary conditions in all directions. The chains were made 'infinite' via binding the ...

Effect of Polyphenylene Sulphide Particles and Films on the Properties of Polyphenylene Sulphide Composites

“…Figure 7 a shows that the energy storage modulus of PPS film composites is higher than that of PPS particle composites throughout the tested temperature range, especially before the glass transition temperature. The energy storage modulus of all samples decreases with the increase in temperature because the mobility of polyphenylene sulphide molecular chains increases with the increase in temperature [ 39 ]. The composite’s energy storage modulus decreases with the percentage of polyphenylene sulphide particles, and the energy storage modulus of all samples decreases with the increase in temperature.…”